Heavy Paper Controlling Scale Deposits in the Pulp & Paper Industry

PULP & PAPER

The pulp and paper industry requires an average of 54 cu meters of water per metric ton of ﬁ nished product. Heavy Paper Controlling scale deposits in the pulp & paper industry

By Jan de Baat Doelman

he pulp and paper industry is one of the heaviest users Processes aff ected by scale formation include: of water within the North American industrial economy, De-inking lines. Water used in this process contains large Trequiring an average of 54 cu meters of water per metric amounts of salts (carbonates; oxalates and silicates of calcium, ton of fi nished product. As water is used in nearly every part of the magnesium and aluminium; and other soluble materials and manufacturing process, accumulation of scale is a phenomenon chemicals). Th ese salts cause scaling within the pipe, fi lters and that can occur in all pulp and paper making processes. Th is occurs pressure screens. One particularly problematic area involves even with the purest water and state-of-the art water treatment. disc fi llers where scale builds up, resulting in a lack of hydraulic Th e scale deposits can cause a number of operational problems pull-through. Th e pipe and fi lter bags need descaling and such as plugging of equipment, ineffi cient usage of chemicals, high-pressure cleaning during plant shutdown, normally every increased utility costs, lost production due to downtime and six months. In addition, cleaners and screen baskets require downgraded products from increased dirt counts. descaling every three months. Blockages frequently occur in screen baskets, showers and pipes, causing a substantial loss Types of Fouling of production. Scale usually refers to an intimate mixture of sparingly soluble Drying cylinders/vacuum pumps. Steam required for heating mineral salts. Mineral scale deposition occurs as a result of heat paper machine drying cylinders causes condensate, which is transfer or pressure changes. Calcium carbonate (CaCO3) scaling collected in separators. Th e vacuum pump draws the incondens- from hard water, and calcium phosphate and oxalate formation are able gases from the top of the main separator to prevent air locks. examples. Other types of fouling include the growth of algae and Limescale buildup occurs in the vacuum pumps, caused by the bacteria (biofouling), the consolidation of loose particles (particu- high temperature of the condensate. late fouling, e.g., from corrosion byproducts), and the accumulation Water treatment plants. Most paper mills have water of “coke”-like deposits (an example of chemical reaction fouling). treatment plants, with many using borehole water, which has

16 ■ JAN/FEB 2014 ■ INDUSTRIAL WATER & WASTES DIGEST The mineral scaling and corrosion prevention system increases the homogeneous precipitation rate of calcium carbonate and other minerals. high levels of calcium carbonate. Despite the use of acid dosing, transfer effi ciency, seized pumps, inoperarable valves, misleading plants generally suff er from scale, necessitating water treatment meter readings and defective heating elements. All of the above alongside the dosing. Not only is the process expensive, but acid processes within the pulp and paper industry have proven treat- dosing also causes corrosion. able with electronic scale removal, saving money on chemicals, Green liquor lines. Regular shutdown of lines due to scale maintenance and downtime. buildup is a regular problem. This often necessitates hydro- Electronic water conditioning. Th e technology works by blasting the lines to remove the hard scale. producing a complex frequency modulated waveform, inducing

Paper liquids. Paper liquids with a CaCO3 concentrate of 10% an electromagnetic fi eld inside the pipe. Scientifi c research shows to 15% are often pumped out from a header tank through a 3-in. that specifi c electromagnetic fi elds initiate colloidal cluster pipe. Th e liquid is diverted into four separate lines with a vibrat- regrouping in the very fi rst stage of crystallization. CaCO3 results ing feeder attached to the end of each line. Th e high concentrate from the interference, which means it no longer has the tendency of CaCO3 means that regular cleaning needs to be carried out, to form a matted structure (fouling or scaling) but becomes scat- sometimes up to twice a day. tered particles that will be fl ushed out by the medium. Air humidifi ers. Standard temperature and humidity are As there is no more scale buildup, the fl owing water will required for stored paper and provided through air humidifi ers. remove existing layers of scale over time. Th is mainly is caused Freshwater is fed to a perforated rotary drum by the humidifi er, by diff usion according to the law of mass action. After electro- vaporized and partially carried away by the airfl ow passing magnetic treatment, hard water is capable of dissolving and through the drums. Residual water drops down and then is removing existing scale layers. reused. Scale buildup on the jets and fi lters necessitates their Electronic water conditioning products aff ect the formation replacement and results in increased energy costs. of scale by increasing the homogeneous precipitation rate of

Black liquor evaporators. Black liquor contains sodium, CaCO3 and certain other minerals. Th e ability to adjust power, carbonate and sulfate. As liquor dry solids increase, so does the frequency and coil confi gurations on site enables performance to concentration of these minerals until their solubility is exceeded. be optimized without downtime or pipe replacement. Salts then crystallize from the liquor and deposit on the heat Th e pulp and paper industry is facing extreme pressure to transfer surfaces of the evaporator. Th e scale produced reduces improve overall operation effi ciency. Cluster rules and other the rate of heat transfer and the evaporation eff ect. environmental factors, in addition to economic factors, are forc- Heat exchangers. Th e deposition of material on heat transfer ing the pulp and paper industry to drive operating systems well surfaces is called fouling, which signifi cantly impacts the thermal beyond their design maximums, exacerbating the problems of and mechanical performance of heat exchangers. Fouling scale buildup and plant shutdown. Th e types, sources and areas increases the overall thermal resistance and lowers the overall where scale can accumulate in pulp and paper mill processes are heat transfer coeffi cient of heat exchangers as well as impedes quite diverse. While more system closures may exacerbate scale fl uid fl ow, accelerating corrosion and increasing pressure drop problems, technologies have been developed that can be used to across the heat exchanger. overcome those problems. Electronic scale removal. Regardless of how hard water eff ects are achieved, the outcome is the same. Scale formation Jan de Baat Doelman is president of Scalewatcher North America results in reduced diameter or blocked pipe, reduced heat Inc. Doelman can be reached at [email protected].